The present invention relates to an X-ray radiography apparatus and, more particularly, to an X-ray cine radiography apparatus.
The X-ray cine radiography uses an image intensifier for converting an X-ray transmitted through an object into a visual image. For obtaining a radiograph with a good contrast, it is essential to take a radiograph at the lowest possible tube voltage, for example, 50 to 70 KV, in order to satisfy a sensitivity characteristic of the intensifier per se, i.e. an energy absorption characteristic of the fluorescent material, for example, cesium iodide (C.sub.s I), on the input side of the intensifier, and an X-ray absorption characteristic of the contrast media. In order to take a cine radiograph with the smallest possible noise arising from a non-uniform distribution of fluorescent particles of the intensifier, it is also considered that a dosage of X-ray onto the intensifier may exceed a usual predetermined one per inch, for example, 15 .mu.R/frame in 9 inch mode. Thus, the radiography at the low tube voltage but of large mAS (the product of a tube current mA and an X-ray dosing time S) is required for a high quality cine radiograph.
The X-ray dosing time S is determined by the number of frames per second and also depends on a dynamic image resolution of the object. For example, a high precise detection of motion in a heart or a blood-vessel may be obtained at several frames per second so that it results in shortening of the dosing time. Since an excessive long dosing time reduces the dynamic image resolution of the object, this fact also restrains the elongation of the dosing time. Thus, in case where the high precision detection of the motion of the object to be radiographed and or the improvement of the dynamic image resolution is desired, the dosing time is preferably short, usually 2 to 5 m sec. For this reason, there is a limit in increasing the mAS value.
In a coronary artery radiography by the X-ray cine radiography apparatus, the X-ray is projected into the object in the frontal or lateral direction for taking a frontal or lateral mode image. This type of radiography apparatus has recently employed an axis-vertical (azimuth) projection which is a projection orthogonal to the axis of a body and an axis-oblique (elevation) projection which is a projection oblique to the body axis in which both the projections are respectively undertaken toward those frontal and lateral directions. In radiographing by the frontal projection, for example, it may be necessary to take successively a plurality of radiographs. Firstly, a positioning is set up in a direction which is orthogonal to the body axis and pointed from the asterior to posterier portion of the human body, as shown in FIG. 1A. A first image (frontal mode image) is radiographed with the positioning, as shown in FIG. 1B. Secondly, a positioning is set up in a direction which is rotated about the axis of the human body by 30.degree. from the direction shown in FIG. 1A, as shown in 2A. With the second positioning, a second image (30.degree. right anterior oblique mode image) is radiographed, as shown in FIG. 2B. A third image (30.degree. right anterior oblique and 30.degree. caudal mode image) is radiographed with a third positioning, as shown in FIG. 3A, set up in a direction which is lowered by 30.degree. from the direction shown in FIG. 2A, and is illustrated in FIG. 3B.
Also in the radiography by the lateral projection, a first positioning, as shown in FIG. 4A, is established in a direction which is clockwise rotated about the axis of the body by 90.degree. from the direction of FIG. 1A. With the first positioning, a first image (lateral mode image) is taken, as shown in FIG. 4B. Then, a second positioning, as shown in FIG. 5A, is established in a direction which is clockwise rotated about the axis of the body by 60.degree. from the direction of FIG. 1A and with the second positioning, a second image (60.degree. lateral anterior oblique mode image) is radiographed, as shown in FIG. 5B. Finally, a third positioning, as shown in FIG. 6A, is selected in a direction which is raised by 30.degree. from the direction in FIG. 5A and with the third positioning, a third image (60.degree. lateral anterior oblique and 30.degree. cranial mode image) is radiographed, as shown in FIG. 6B.
The dosage of X-rays in the radiography, as mentioned above, which requires a plurality of the positionings including the oblique directional projections, is two to three times as large as that in the conventional radiography by the single frontal or lateral projection.
The X-ray dosage necessary for the multi-positioning radiography is unattainable even if the large-capacity X-ray tube currently available is used, which is preferably operated at low voltage in the X-ray cine radiograph apparatus. This impells us to use the apparatus at high X-ray tube voltage. For this reason, the conventional cine radiograph apparatus provides a cine radiograph with poor contrast.
As described above, the conventional X-ray cine radiograph apparatus has a limit in increasing the tube current X-ray dosage product mAS and is unsuitable for the appliance of a large dosage of X-rays.